A serious disease occurred in many Haliotis diversicolor Reeve farms during 1999 and 2002. The symptoms of the infected abalone were as follows: the pleopod became stiff, its surface became black, and the mantle shrank. All sizes of abalone became infected and mortality was high. A spherical virus was detected in the tissues of the digestive gland, mantle, gill, and intestine of infected animals. It was observed in the cytoplasm and had a diameter about similar to 100-130 nm. Electron microscope examination showed that organelles of infected cells, such as the endoplasmic reticulum; expanded, mitochondria dilated, nuclear membranes dissolved or disappeared, and the edges of the nucleolus swelled. The pathogenicity of the virus was confirmed by experimental infection.